Wnt (pronounced “wint”) proteins are a family of cysteine-rich, secretory glycoproteins of approximately 40 kDa, and are known to be involved in various cell developmental processes including cell polarity (Moon R T et al., Science, 2002; Reya T and Clevers H, Nature, 2005). In humans, 19 wnt proteins have been reported, and 10 frizzled proteins as Wnt receptors and 2 coreceptors (LPR 5 and 6) are known (He X C et al., Nat Genet, 2004; Tamai K et al, Mol Cell, 2004; Tamai K et al., Nature, 2000).
Typical Wnt signaling induces stabilization and accumulation of cytoplasmic 3-catenin through the regulation of protein kinase complex, translocation of β-catenin into the nucleus where it acts as a transcription activivator. This transcription activity is reported to be caused by transcription factors in the group of the Lefl/Tcf (Moon R T et al., Science, 2002; Reya T and Clevers H, Nature, 2005; Wodarz A and Nusse R, Annu Rev Cell Dev Biol., 1998).
In the absence of a Wnt signal, β-catenin is phosphorylated by GSK-3β complex (glycogen synthase kinase 3β), which is composed of the protein kinases, GSK-3β and casein kinase I (CKI), axin, Dsh (dishevelled) protein, and APC (Adenomatous Polyposis Coli). The phosphorylation of β-catenin leads to the production of the DpSGXXpS motif (where pS stands for phosphorylated serine and X for any amino acid), which is recognized by β-TrCP containing the F-box which is a kind of the E3/SCF ubiquitin ligase. After being ubiquitinated, β-catenin undergoes proteasomal degradation, thus Lefl/Tcf-mediated transcriptional activity is inhibited (Hart M et al., Curr Biol., 1999; Winston J T et al., Curr Biol., 1999).
Meanwhile, when Wnt proteins bind to the frizzled receptors and the coreceptor LRP5, the activity of the GSK-3β complex loses its ability to induce phosphorylation of β-canetin, resulting in the promotion of transcription of target genes by association with Lefl/Tcf proteins (Reya T and Clevers H, Nature, 2005; Tamai K et al., Mol Cell, 2004; Westendorf J J et al., Gene, 2004).
Mutation of the proteins involved in the Wnt signal transduction system is closely correlated with various human diseases such as abnormalities in development, hair follicle morphogenesis, stem cell differentiation and cell proliferation, and particularly, is also believed to be related to oncogenesis, such as colorectal cancer and leukemia (Taipale et al., Nature, 2001). In addition, it has been reported that the Wnt signal transduction system plays a crucial role in the differentiation and development of nerve cells for the central nervous system, suggesting a relationship between Wnt proteins and the incidence of various diseases of the central nervous system, including neurodegenerative diseases and depression. Particularly, it is also found that Wnt signaling is related to diseases resulting from the abnormality of nerve cells, such as brain damage, Parkinson's disease, stroke, epilepsy, Alzheimer's disease, depression, bipolar disorder, and schizophrenia. Thus, to treat these diseases, it requires to be substituted with healthy nerve cells which operate normally, and as an alternative, the control of Wnt signaling was suggested (Dieter-Chichung Lie et al., Nature 2005).
Further, according to the recent research, Wnt proteins have been found to be significantly involved in the differentiation of adult stem cells into adipocytes or osteoblasts. Firstly, it is found that persons with non functional LRP5, a Wnt coreceptor, undergo a abnormally significant decrease in bone density (Boyden L M et al., N Engl J Med., 2002; Gong Y et al., Cell, 2001). From studies with Wnt10b transgenic mice, which were manipulated to be highly expressed specifically in adipose and bone marrow cells, it was observed they were remarkably increased in bone density, whereas decreased in obesity and glucose intolerance by high adipose ingestion (Longo K A et al., J Biol. Chem., 2004; Bennett C N et al, Proc Natl Acad Sci USA, 2005). Also, in case of the overexpression of Wnt10b in adult stem cells, the level of osteoblast-specific markers is found to be increased (Bennett C N et al, Proc Natl Acad Sci USA, 2005). In contrast, Wnt10b-knockout mice were measured to have significantly decreased levels of osteoblasts and bone density.
Additionally, it is reported that β-catenin itself plays an essential role in postnatal bone acquisition through knockout mice model (Holmen et al., J Biol. Chem., 2005).
While Stably activated β-catenin acts to increase the expression of the bone-specific alkaline phosphatase, a marker of early-stage osteoblast differentiation, it does not effect on the expression of osteocalcin, a marker of late-stage osteoblast differentiation (Vinals F et al., FEBS Lett., 2002). Moreover, Lefl and β-catenin may inhibit Runx2-dependent transcriptional activation (Kahler R A and Westendorf J J, J Biol. Chem., 2003). While Wnt10b signaling transduced along the typical Wnt pathways, it has a signal mechanism, which is dependent on Wnt/GSK-3b but not dependent on β-catenin, in the regulation of the production of osteoblasts and adipocytes (Vinals F et al., FEBS Lett., 2002; Kahler R A and Westendorf J J, J Biol. Chem., 2003). Therefore, it has been suggested that there exist β-catenin-independent mechanisms.
On the basis of the significance of the above-mentioned Wnt signal transduction pathways, Wnt signaling molecules are considered as targets for drug screening to develop medications for curing Wnt-related diseases, such as breast cancer, colorectal cancer, metabolic bone diseases, obesity, etc. Extensive efforts have been made to find new regulators, either activators or inhibitors, and develop them into medications.
Recently, also, the Wnt signaling pathway has been reported to play an important role in the maintenance, differentiation and proliferation of stem cells (Reya T et al., Nature., 2003; Trowbridge J J et al., NatuerMed., 2006). Thus, extensive research has been directed to the development of promoter for tissue regeneration, control of hair loss, haematopoiesis, and stimulation of stem cell growth, maintenance and differentiation.
In modern times, bone diseases is increasing due to socioenvironmental and genetic factors, particularly due to increase of population of elderly persons. Generally, bone diseases occur and develop without special symptoms, and rapidly worsen with age. Although many drugs have been developed for the treatment of bone diseases thus far, most of them mainly aim to alleviate pain or to retard the decrease of bone density. They are not effective as a curative medication which aims for increasing the bone density of patients who suffer from osteoporosis. Some other drugs are usually in the form of injections and are reported to produce side effects upon long-term administration thereof.
Therefore, there is a need for novel drugs that effectively treat bone diseases without the problems mentioned above.